Say I need to buy some retzuot or tzitzit strings and for some reason or another I don't know whether my family accepts avodat mechona or not. Is it typically acceptable to rule leniently according to Western European sources?

Some background information: I have asked my father about this as I need new retzuot for my tefillin, however, he has never personally been involved in the purchase of tefillin, his father having gotten him his and my rabbi having gotten me mine.